1. Field of the Invention
The invention relates to ceramic materials for thermal barriers coating systems in high temperature cycling applications, and more particularly to ultra-pure zirconia- and/or hafnia-based materials for use in thermal barrier coating applications.
2. Description of the Related Art
Gas turbine engines are widely used for aircraft propulsion and for ground based power generation. In order to increase efficiency, gas turbines are required to run hotter and faster. Therefore, there is a continued demand to increase firing temperatures in the combustion portions of gas turbines, which provides one of the greatest materials challenges. The development of superalloys has led to an increase in the hot-section operation temperature of gas turbine engines from approximately 760° C. to 1040° C. over the period 1940-1970. Since 1970, further improvement of gas-turbine engine performance has become increasingly difficult because conventional nickel- or cobalt-based superalloys have already reached their maximum temperature capabilities. Since NASA proposed to use a thin layer of insulating ceramic to help shield components from direct exposure to high temperatures on space vehicles and rocket engines in the late 1950s and early 1960s, extensive research and development on thermal barrier coatings (TBCs) has been performed and funded by government agencies, research institutions and industries.
Today, TBCs are widely used in gas turbines. In order to function as a thermal barrier, TBC materials must meet the following requirements: (1) low thermal conductivity; (2) high coefficient of thermal expansion; (3) high melting point; (4) high thermal shock resistance; and (5) be resistant to erosion, (6) compatibility with bond coat. When all these requirements are considered, 6˜9 weight percent yttria stabilized zirconia (7YSZ) is the conventional material of choice. The thermal conductivity of 7YSZ TBCs deposited using air plasma spray can be as low as 0.8 W/(Km). However, the thermal conductivity can go up to 1.5˜2.0 W/(Km) after high temperature exposure as a result of sintering, which significantly deteriorate the thermal insulation capability of TBCs. In addition, the elimination of microcracks and fine void due to sintering leads to the increase of coating stiffness, which has an adverse effect on coating durability. Accordingly, there is a quest to find new materials and to optimize coating structures so as to produce a TBC that has prolonged durability and can provide excellent thermal insulation over extended period of service time.